1. Field of the Invention
The present invention relates to a boiling water reactor core including a water rod, a boiling water reactor including the core, and a method of operating the boiling water reactor.
2. Description of the Related Art
Usually, a core of a boiling water reactor (BWR) includes a number of fuel assemblies arrayed in the form of a square lattice. Each of the fuel assemblies comprises a number of fuel rods arrayed in the form of a square lattice, and at least one water rod disposed near a center position of the array of the fuel rods. A channel box surrounds an outer periphery of a fuel bundle of the fuel rods held together by a fuel spacer; The fuel rods each include a number of fuel pellets filled in a cladding pipe made of an zirconium alloy, for example. Most of the fuel pellets are each made up of enriched uranium having an increased concentration of U-235 that is a fissile material.
The water rod is a hollow pipe which takes in water as a coolant through an opening formed in its lower portion, and allows the water to flow out through an opening formed in its upper portion. This water circulation increases a neutron moderating effect in a central portion of the fuel assembly in horizontal section, and enhance nuclear fission in the central portion. Various structures of the water rod have been proposed in the past. Recently, a water rod including a rising pipe and a falling pipe communicated with the rising pipe has been proposed. Such water rods are disclosed in JP, A, 63-73187, U.S. Pat. No. 5,023,047, U.S. Pat. No. 5,640,435, and Hitachi Hyoron, Vol. 74, No. 10 (1992), pp. 55-60 (especially FIG. 5 in page 58). The rising pipe has a lower end opened below an upper surface of a lower tie plate which holds lower end portions of fuel rods. The falling pipe has a lower end opened above the upper surface of the lower tie plate. In the disclosed water rods, a difference between a pressure at the lower end opening of the rising pipe and a pressure at the lower end opening of the falling pipe is equal to a pressure loss occurred in a flow passage above the upper surface of the lower tie plate. During reactor operation, therefore, the surface of water in the rising pipe takes a level that gives a density head corresponding to the pressure loss, and the level of the water surface changes vertically depending on an increase and decrease in flow rate of cooling water passing through a core (i.e., in core flow rate). The above-mentioned water rods are called spectral shift rods.
A nuclear reactor is shut down to replace a part of fuel assemblies in a reactor core. During shutdown of the reactor core, the fuel assemblies whose lifetime has expired are taken out of the core, and new fuel assemblies are loaded in the core. A period of reactor operation from start-up of the reactor after loading of the new fuel assemblies to next shutdown of the reactor for replacement of the fuel assemblies is called a operation cycle.
Because of the loading of the new fuel assemblies, excess reactivity is increased at the beginning of the operation cycle. Control of the excess reactivity is important to keep constant a reactor power at the rated power (100% power) in the operation cycle. The excess reactivity is controlled by a burnable poison filled in a part of fuel rods and by insertion of control rods into the core. In the case of using a spectral shift rod, however, the excess reactivity is controlled by adjusting the level of the water surface in the rising pipe instead of manipulating the control rods. At the start-up of the reactor, the control rods are all withdrawn out of the core.
During the rated power operation at the early period of the operation cycle, the core flow rate is relatively small and the water surface is formed in the rising pipe. In this condition, the void fraction increases in upper portions of the fuel assemblies, and nuclear fission is suppressed to hold down the excess reactivity. Approaching the end of the operation cycle, the core flow rate increases and the level of the water surface in the rising pipe rises. This phenomenon is equivalent to a lowering of the void fraction. Nuclear fission is more activated with a rise of the level of the water surface. At the time of reaching a certain point in the operation cycle, the rising pipe is filled with water.
The above-cited JP, A, 63-73187, U.S. Pat. No. 5,023,047, U.S. Pat. No. 5,640,435, and Hitachi Hyoron, Vol. 74, No. 10 (1992) describe adjustment of the core output during the rated power operation. At the beginning of the operation cycle, the neutron moderating effect is reduced and U-235 is less consumed. At the same time, U-238 occupying a large part of the fuel material absorbs fast neutrons, whereupon Pu-239 is produced. Filling the rising pipe with the water in the late period of the operation cycle to enhance the neutron moderating effect promotes nuclear fission of U-235 and Pu-239. Consequently, production of Pu-239 is promoted and nuclear fission of Pu-239 is developed in the late period of the operation cycle, thus resulting in a saving of U-235 (uranium conservation).
In each operation cycle of a BWR, the core flow rate and the control rod manipulation are controlled in an automatic manner during a period from an low end at the automatic flow control range (described later), which represents a setting flow rate with respect to the core flow rate, until the reactor core power reaches the rated power. When the core flow rate is smaller than the low end at the automatic flow control range, the core flow rate and the control rod manipulation are controlled in a manual manner. In a predetermined range of the core flow rate from the rated power operation to the low power operation (hereinafter referred to the non-rated power operation), the core flow rate can be automatically controlled. At a low end of the automatic flow control range corresponding to the non-rated power operation (i.e., at the low end at the automatic flow control range), it is required to satisfy certain minimum restrictions of the core preset on the nuclear thermal-hydraulic stability from the standpoint of ensuring safety.
The above-cited JP, A, 63-73187, U.S. Pat. No. 5,023,047, U.S. Pat. No. 5,640,435, and Hitachi Hyoron, Vol. 74, No. 10 (1992) primarily intend to adjust the power during the rated power operation, namely to vertically change the level of the water surface in the rising pipe during the rated power operation. As an incidental advantage, the spectral shift rods described in those known references have a possibility that the nuclear thermal-hydraulic stability at the low end at the automatic flow control range during the non-rated power operation is slightly improved. Those spectral shift rods have however drawbacks below.
Because of a water surface being present in the rising pipe during the rated power operation, supposing if the core flow rate should abruptly increase due to, e.g., an abnormal condition occurred in a pump control system, this would possibly cause an abrupt rise of the water surface and abruptly increase an amount of water in the reactor core, thus resulting in a difficulty in suppressing a rising rate of the reactor power. In other words, there has been such a risk that influences of a transient event such as an increase of the core flow rate cannot be always avoided with sufficient reliability.
Additionally, U.S. Pat. No. 4,708,846 discloses, though not a spectral shift rod, a water rod having a rising pipe and a falling pipe which are communicated with each other. The disclosed water rod primarily intends to improve an ability of cooling fuel rods with water flowing out of an outlet of the falling pipe. The height from a lower end of the fuel effective length to the outlet of the falling pipe is set to be not less than 65% but not more than 75% of the fuel effective length.
An object of the present invention is to provide a boiling water reactor core, a boiling water reactor, and a method of operating the boiling water reactor which can suppress influences of a transient event during the rated power operation, and can improve the nuclear thermalhydraulic stability during the non-rated power operation.
To achieve the above object, a first aspect of the present invention resides in a core of a boiling water reactor including fuel assemblies each comprising a plurality of fuel rods having a fuel effective length L, at least one water rod, an upper tie plate for holding upper end portions of the fuel rods and the water rod, a lower tie plate including a fuel holding portion to hold lower end portions of the fuel rods and the water rod, and a channel box surrounding an outer periphery of the fuel rods tied up into a bundle: and fuel support pieces for supporting the lower tie plates of the fuel assemblies, wherein the fuel support piece includes a first coolant passage formed therein and having an orifice with an inner diameter of about 6.2 cm; the fuel holding portion has a plurality of through holes for introducing a coolant in the lower tie plate to a second coolant passage defined between the fuel rods above the fuel holding portion, a total cross-sectional area S1 of all the through holes being smaller than a total cross-sectional area S2 of the second coolant passage; the water rod includes a rising passage opened to a space in the lower tie plate below the fuel holding portion and introducing upward the coolant introduced to the rising passage, and a falling passage communicated with the rising passage and introducing downward the coolant introduced through the rising passage, the falling passage having a coolant outlet opened to the second coolant passage above the fuel holding portion; the relationship of 0.2xe2x89xa6rxe2x89xa60.4 holds on an assumption that a ratio S1/S2 of the total cross-sectional area S1 to the total cross-sectional area S2 is r; and a height h from an upper surface of the fuel holding portion to the coolant outlet is set to satisfy the relationship of: xe2x88x922.1r2+2.2rxe2x88x920.3xe2x89xa6(h/L) less than xe2x88x922.2r2+1.8r+0.04.
To achieve the above object, a second aspect of the present invention resides in a core of a boiling water reactor including fuel assemblies each comprising a plurality of fuel rods having a fuel effective length L, at least one water rod, an upper tie plate for holding upper end portions of the fuel rods and the water rod, a lower tie plate including a fuel holding portion to hold lower end portions of the fuel rods and the water rod, and a channel box surrounding an outer periphery of the fuel rods tied up into a bundle; and fuel support pieces for supporting the lower tie plates of said fuel assemblies, wherein the fuel support piece includes a first coolant passage formed therein and having an orifice with an inner diameter of about 5.6 cm; the fuel holding portion has a plurality of through holes for introducing a coolant in the lower tie plate to a second coolant passage defined between the fuel rods above the fuel holding portion, a total cross-sectional area S1 of all the through holes being smaller than a total cross-sectional area S2 of the second coolant passage; the water rod includes a rising passage opened to a space in the lower tie plate below the fuel holding portion and introducing upward the coolant introduced to the rising passage, and a falling passage communicated with the rising passage and introducing downward the coolant introduced through the rising passage, the falling passage having a coolant outlet opened to the second coolant passage above the fuel holding portion; the relationship of 0.2xe2x89xa6rxe2x89xa60.4 holds on an assumption that a ratio S1/S2 of the total cross-sectional area Si to the total cross-sectional area S2 is r; and a height h from an upper surface of the fuel holding portion to the coolant outlet is set to satisfy the relationship of: xe2x88x924.2r2+3.4rxe2x88x920.4xe2x89xa6(h/L) less than xe2x88x920.53r2+0.5r+0.46.
A coolant having entered the first coolant passage in the fuel support piece is introduced to the interior of the lower tie plate, passes the plurality of through holes formed in the fuel holding portion of the lower tie plate, and then flows into the second coolant passage to cool the fuel rods. Because a pressure loss is developed by the through holes in such a flow of the coolant, the pressure of the coolant in the second coolant passage is lower than the pressure of the coolant in the lower tie plate.
A part of the coolant having entered the first coolant passage rises in the rising passage of the water rod, falls in the falling passage thereof, and then flows out from the cooling outlet into the second coolant passage. By making the rising passage opened below the fuel holding portion, the pressure loss caused by the through holes in the fuel holding portion is negligible for the flow of the coolant flowing into the rising passage of the water rod. Therefore, a surface of the coolant is formed in the rising passage at a level corresponding to a pressure difference resulted from the pressure loss. More specifically, assuming that a height from an upper surface of the fuel holding portion to the coolant surface formed in the rising passage is H, the pressure at the coolant inlet of the rising passage is P1, the pressure at the coolant outlet of the falling passage is P2, the density of the coolant in the rising passage is xcfx81, and the acceleration of gravity is g, the height H is expressed by;
H=(P1xe2x88x92P2)/(xcfx81xc3x97g)
Here, since the pressure P2 is reduced as the height h increases, the height H increases as the height h increases. The height H also varies depending on the ratio r of the total cross-sectional area S1 to the total cross-sectional area S2. In other words, as the ratio r becomes larger, the throttling effect developed by the through holes in the fuel holding portion reduces and the pressure loss also reduces. An increasing rate of the height H with respect to the height h is therefore reduced.
In view of the above, according to the first aspect of the present invention, the height h is set to satisfy the relationship of:
xe2x88x922.1r2+2.2rxe2x88x920.3xe2x89xa6h/L (0.2xe2x89xa6rxe2x89xa60.4)
and according to the second aspect of the present invention, the height h is set to satisfy the relationship of:
xe2x88x924.2r2+3.4rxe2x88x920.4xe2x89xa6h/L (0.2xe2x89xa6rxe2x89xa60.4)
By so setting, during the rated power operation of the reactor, the height H of the coolant surface in the rising passage can be made not less than 3.7 m that is the fuel effective length L. Accordingly, a region in the rising passage corresponding to at least the fuel effective length can be fully filled with the coolant. If there should occur a transient event such as an abrupt increase of a flow rate of the coolant supplied to the core due to, e.g., an abnormal condition occurred in a pump control system, a rising rate of the reactor power would be small because the region in the rising passage corresponding to the fuel effective length is originally fully filled with the coolant. As a result, the first and second aspects of the present invention can suppress influences of the transient event during the rated power operation.
Further, according to the first aspect of the present invention, the height h is set to satisfy the relationship of:
h/L less than xe2x88x922.2r2+1.8r+0.04 (0.2xe2x89xa6rxe2x89xa60.4)
and according to the second aspect of the present invention, the height h is set to satisfy the relationship of:
h/L less than xe2x88x920.53r2+0.5r+0.46 (0.2xe2x89xa6rxe2x89xa60.4)
By so setting, at least at the low end at the automatic flow control range during the non-rated power operation in which the reactor power is lower than that during the rated power operation, a surface of the coolant is formed in the region in the rising passage corresponding to the fuel effective length, and a vapor zone is formed in the rising passage above the coolant surface. Therefore, the void fraction in the core is increased to reduce the neutron moderating effect, whereby the reactor power is suppressed. As a result, the nuclear thermal-hydraulic stability of the core during the non-rated power operation is improved.
To achieve the above object, a third aspect of the present invention resides in a core of a boiling water reactor including fuel assemblies each comprising a plurality of fuel rods having a fuel effective length L, at least one water rod, an upper tie plate for holding upper end portions of the fuel rods and the water rod, a lower tie plate including a fuel holding portion to hold lower end portions of the fuel rods and the water rod, and a channel box surrounding an outer periphery of the fuel rods tied up into a bundle; and fuel support pieces for supporting the lower tie plates of the fuel assemblies, wherein the fuel support piece includes a first coolant passage formed therein and having an orifice with an inner diameter in the range of not less than 5.6 cm but not more than 6.2 cm; the fuel holding portion has a plurality of through holes for introducing a coolant in the lower tie plate to a second coolant passage defined between the fuel rods above the fuel holding portion, a total cross-sectional area S1 of all the through holes being smaller than a total cross-sectional area S2 of the second coolant passage; the water rod includes a rising passage opened to a space in the lower tie plate below the fuel holding portion and introducing upward the coolant introduced to the rising passage, and a falling passage communicated with the rising passage and introducing downward the coolant introduced through the rising passage, the falling passage having a coolant outlet opened to the second coolant passage above the fuel holding portion; the relationship of 0.2xe2x89xa6rxe2x89xa60.4 holds on an assumption that a ratio S1/S2 of the total cross-sectional area S1 to the total cross-sectional area S2 is r; and a height h from an upper surface of the fuel holding portion to the coolant outlet is set to satisfy the relationship of: xe2x88x924.2r2+3.4rxe2x88x920.4xe2x89xa6(h/L) less than xe2x88x922.2r2+1.8r+0.04.
With the third aspect, influences of the transient event during the rated power operation can be surely suppressed over the entire range of the inner diameter of the orifice from about 5.6 cm to about 6.2 cm, and the nuclear thermal-hydraulic stability of the core during the non-rated power operation can be improved.
To achieve the above object, a fourth aspect of the present invention resides in a method of operating a boiling water reactor wherein the rising passage is filled with the coolant during a period of rated power operation, and a surface of the coolant is formed in the rising passage during a period of non-rated power operation in which a flow rate of the coolant supplied to the fuel assemblies is lower than that during the period of rated power operation.
With the above operating method, influences of the transient event during the rated power operation can be suppressed, and the nuclear thermal-hydraulic stability of the core during the non-rated power operation can be improved.